1. Field of the Invention
The present invention relates to a particle counter for measuring the concentration (quantity) and the particle-size distribution of minute particles (organic substances, dust, bacteria and the like) contained in a fluid. For example, the present invention may be used to control a fluid such as ultrapure water used in a process for washing wafers in the production of semiconductors and clean air in a clean room.
In particular, the present invention relates to a particle counter for counting minute particles in a fluid that is flowing at an appointed flow velocity and that has a substantially uniform flow velocity distribution comprising an irradiating optical system for irradiating a central portion of said fluid with a laser beam from the side and perpendicular to the flow direction of the fluid, and further comprising a detecting optical system for detecting light from the laser beam that is scattered by the minute particles contained in said fluid.
2. Description of Related Art
One example of a prior art particle counter is roughly shown in FIG. 4. A fluid S to be measured is flown through a flow cell (not shown) at an appointed velocity of flow by means of a nozzle 1. An irradiating optical system (a) is provided for irradiating with a laser beam a central portion, where the flow velocity distribution is uniform as far as possible, in the fluid S to be measured, from a side in a direction y (hereinafter referred to as the second direction) meeting at right angles with a flow direction z of the fluid S to be measured (hereinafter referred to as the first direction). A detecting optical system (b) is provided for detecting, from a side in said first direction z, light L' scattered by minute particles contained in said fluid S so as to measure the concentration and the particle-size distribution of the minute particles contained in said fluid S.
The irradiating optical system (a) typically includes a cylindrical lens 2 for turning said laser beam L into a flat beam (short in said first direction z and long in a direction x, hereinafter referred to as the third direction, meeting at right angles with said first direction z and said second direction y). The detecting optical system (b) is provided with a photoreceiving lens 3 and a slit member 5' having a slit 4' for delimiting a detecting range (r) by limiting an incidence of said scattered light L' in said second direction y and said third direction x at an image-forming position (position forming a real image) thereof.
However, in order to ensure the desired resolution power (in particular the particle-size resolution power) with a particle counter of the above-described conventional construction, it is necessary to conduct the measurement using only that portion of the laser beam L where the intensity of the irradiated laser beam L is almost uniform as far as possible (the vicinity of the peak at the center of the Gaussian distribution in the third direction x corresponding to the hatched portion schematically shown in FIG. 5).
In order to sufficiently delimit the source of the scattered light L', the slit member 5' of the conventional detecting optical system (b) must be provided with a remarkably narrow slit 4' which limits the detection range (r) not only in the irradiating direction of the laser beam L (second direction y), but also in the radial direction of the laser beam L (third direction x, refer to FIG. 6). Moreover, the scattered light L' must be detected from a side that is in the flow direction (first direction) z of the fluid S. Accordingly, the following disadvantages are present in the above-described prior art device:
(A) Since the detection range (r) is delimited by limiting the incident scattered light L' by means of the slit member 5' as shown in FIG. 6, a reduction in resolution power is inherent, since an outer edge portion of an image I required for the aberration of the detecting optical system (b) is cut in all circumferential portions of the slit 4'. Thus, the resolution power is fundamentally inferior and, accordingly, the aberration must be remarkably reduced by, for example, providing the detecting optical system (b) with a specialized and expensive aberration-compensating means;
(B) The slightest change in the refractive index of the fluid S leads to the dislocation of a focus in the slit member 5', whereby the undesirable phenomenon of cutting the outer edge portion of the image I by the circumferential or perimeter portions of the slit 4' is further aggravated. Thus, the slit member 5' must be repeatedly moved in order to regulate the focus, and the resolution power is still further reduced. For example, a specialized microfocus regulating means capable of precisely moving the slit member 5' dependent upon the change of the refractive index of the fluid S must be provided. This results in an expensive complication in the construction of the detecting optical system (b);
(C) As described in Paragraph (A) above, the aberration of the detecting optical system (b) must be reduced in order to ensure the resolution power. However, it is difficult to increase an openness, and the quantity of the scattered light L' received so as to improve the S/N ratio is deteriorated, while simultaneously accounting for aberration of the detecting optical system (b); and
(D) Because the detecting optical system (b) must be provided on the side in the flow direction (first direction) z of the fluid S to be measured, although not shown in FIG. 4, the construction is complicated and overly large in that the fluid passage on the downstream side of the measuring portion must be bent, or a whirling flow must be used. Moreover, the control of flow velocity is apt to become difficult.